The cellular response in small bowel to removal of the colon and the response in colon to removal of the small bowel will be studied in rats. Preliminary evidence suggests a major increment in the hyperplastic activity of small bowel within five days of total colectomy, rather than a hypertrophic response at two months as usually described. Assessments of proliferative activity will be made by measurements of nucleic acid synthesis, labeling indices, and cell migration rate. When the kinetics are defined, effects of age and neoplasia will be determined. Rats from the weanling to senile states will be used. Dimethylhydrazine will be the usual carcinogen. Azathioprine will be employed as a reversible suppressor of DNA synthesis.